Technical Field
The present invention relates to a working machine on which a power storage device is mounted.
Description of the Related Art
In the related art, a working machine which drives an electric motor by electric energy charged in a power storage device is disclosed. A state of charge SOC of the power storage device is used for controlling the working machine. When an electric double layered capacitor, a lithium ion capacitor, or the like is used in the power storage device, the state of charge SOC is represented by the square of a ratio between an open circuit voltage and a maximum rated voltage of the power storage device.
In order to remove influence of voltage drop due to an internal resistance to measure the open circuit voltage of the power storage device, a charge-discharge current of the power storage device should be 0. Accordingly, when the charging and discharging of the power storage device is performed, the open circuit voltage of the power storage device cannot be measured. Instead of a measured value of the open circuit voltage, the state of charge SOC of the power storage device can be obtained using a calculation value of the open circuit voltage which is obtained by using measured values of the internal resistance and an inter-terminal voltage, and a measured value of the charge-discharge current.
The internal resistance used in the calculation of the state of charge SOC is obtained at the time of starting of the working machine. Hereinafter, a method for obtaining the internal resistance will be described. When the working machine is started, the open circuit voltage is measured in a state where the charge-discharge current of the power storage device is set to 0. Thereafter, charging to the power storage device starts, and the inter-terminal voltage and the charging current are measured. The internal resistance can be obtained using the measured open circuit voltage, the inter-terminal voltage, and the charging current.
In the related art, a method of correcting the open circuit voltage according to an ambient temperature is disclosed. A charging rate of the power storage device is obtained using the corrected open circuit voltage.